Chapter 23

Plant Structure and Function

Section 23-1Specialized Tissues in Plants

Structure of a Seed Plant 3 organs -> roots,

stems, leaves Linked by tissues

that provide support, protection, nutrient production and transport

Structure of a Seed Plant Roots

Anchor plants, prevent erosion, absorb nutrients/water and transport them, store food, hold plants upright

Stems Produce leaves/reproductive structures,

contain transport systems Leaves

Photosynthesis, have adjustable pores to reduce water loss and help gas exchange

Plant Tissue Systems 3 main tissue systems -> dermal, vascular,

and ground Dermal Tissue - protective, outer covering

Single cell layer in young plants called epidermis, the outer surface often covered with a waxy cuticle

In older plants usually many layers, sometimes covered with bark

Some epidermal cells have trichomes, which protect leaves and give them a fuzzy appearance

In roots, incudes root hairs to absorb water

Plant Tissue Systems Vascular Tissues –

support plant bodies, transport water and nutrients Xylem – transports

water Phloem –

transports products of photosynthesis

Xylem Cells called tracheids As they mature, they die and leave their

cell walls which contain lignin (gives wood strength)

Cells have connecting openings for water to pass

Pits allow water to diffuse into ground tissue

Xylem Angiosperms have

second xylem tissue called vessel elements – wider than tracheids, arranged end to end

Mature and die, cell walls develop slits at each end for water to move freely

Phloem Alive at maturity Main cells called sieve tube elements,

arranged end to end forming sieve tubes

Small holes at ends so nutrients can move from cell to cell

Lose nuclei and most organelles as they mature

Phloem Companion cells

surround sieve tube elements - keep nuclei/organelles

Plant Tissue Systems Ground tissue – produces/stores

sugars, contributes to physical support Parenchyma cells – thin walls, large

central vacuole surrounded by thin layer of cytoplasm – chloroplasts in leaves

Collenchyma cells – thicker walls, flexible, provide support

Sclerenchyma cells - thickest walls, rigid, makes up seed coat

Parenchyma Collenchyma Sclerenchyma

Plant Growth and Meristems Meristems –

regions of unspecialized cells in which mitosis produces new cells ready for differentiation

Apical meristems found in places of rapid division – tips of stems and roots

Plant Growth and Meristems At first, cells produced in apical

meristems are all thin, unspecialized Gradually mature and differentiate to

form each tissue system Meristems also create highly specialized

cells of cones and flowers Patterns of gene expression changes

the stem’s apical meristem

Section 23-2Roots

Root Structure and Growth As soon as a seed sprouts, its first root

brings in water/nutrients from soil Cells divide rapidly, pushing root tips

into soil, providing raw materials for developing stems and leaves

Root Structure and Growth Taproot Systems

Primary root grows long and thick (taproot) giving rise to smaller branches

Can store sugars and starches Fibrous Root Systems

Begin with one primary root, which is replaced by many equally sized branches that grow separately from the base of the stem

Help prevent soil erosion

Dandelion (taproot) Grass (fibrous root)

Anatomy of a Root Epidermis made of dermal tissue –

protection and absorption Surface covered in root hairs – penetrate

between soil particles and increase surface area

Cortex composed of ground tissue Water/minerals move from epidermis Stores products of photosynthesis and

starches

Anatomy of a Root Endodermis – layer of ground tissue

enclosing vascular cylinder – moves water and minerals to center of root

Vascular cylinder in the center composed of xylem and phloem Dicot roots have central column of xylem

Anatomy of a Root Apical meristems near root tip allow

roots to increase in length Root cap protects meristem, secretes

slippery substance to ease progress through soil Cells at tip scraped away and replaced

continually

Root Functions Uptake of plant nutrients

Soil contains sand, silt, clay, air, bits of decaying animal/plant tissue in varying amounts

Plants need inorganic nutrients like nitrogen, phosphorus, potassium, magnesium, calcium

Trace elements also important, but excessive amounts can be toxic

Root Functions Active transport of dissolved nutrients

Active transport proteins in root hairs, other epidermal cells

Bring in mineral ions from soil Water movement by osmosis

Mineral ions accumulate in root, water “follows”

Root Functions Movement into vascular cylinder

Move through cortex Cylinder enclosed by endodermis – cells

meet and cell walls from waterproof zone called Casparian strip

Casparian strip forces water/minerals to move through cell membrane rather than between cells – filter and control water

Ensures one-way flow

Root Functions Root pressure

Minerals pumped into vascular cylinder, water follows by osmosis creating pressure

Water has to go up - root pressure forces water through vascular cylinder into the xylem

Up and up!

Section 23-3Stems

Stem Function Produce leaves, branches and flowers Hold leaves up to sun Transport substances

Xylem and phloem form continuous tubes from roots to stems to leaves

In many plants they function in storage and aid in photosynthesis

Anatomy of a Stem Surrounded by layer of epidermal cells

with thick cell walls and a waxy protective coating

Anatomy of a Stem Growing stems have

nodes where leaves are attached

Buds contain apical meristems to produce new stems and leaves

Larger plants have woody stems to support leaves and flowers

Monocot Stems Vascular

bundles (clusters of xylem and phloem) scattered throughout ground tissue composed mainly of parenchyma cells

Dicot Stems Vascular bundles

arranged in a ring pattern

Parenchyma cells inside ring called pith, outside form cortex

Complexity increases as stem increases in diameter

Primary Growth Occurs in all seed plants – apical

meristems increase plant length

Secondary Growth Larger plants require older parts of stem

to increase in thickness Common in dicots and gymnosperms

Secondary Growth Takes place in meristems called

vascular cambium (produced vascular tissues, increase thickness of stem) and cork cambium (outer covering)

Growth from Vascular Cambium Thin layer of cells between xylem and

phloem Xylem pushed in, phloem pushed out Increases diameter of stem each year

Wood Formation Layers of secondary xylem produces by

vascular cambium Older xylem near center no longer

carries water – heartwood (dark) Surrounded by sapwood – active in

fluid transport (light)

Tree Rings In spring, vascular cambium produces

light colored rings of xylem (early wood) Cells grow less as season continues,

have thicker cells walls, darker in color (late wood)

A ring = a year of growth Thick rings mean favorable weather

Formation of Bark Everything outside the vascular

cambium in a mature stem (phloem, cork cambium, cork)

Expansion leads to oldest tissue splitting Cork cambium surrounds cortex

producing a thick layer of cork to prevent water loss

Outer layers may flake off as stem thickens

Section 23-4Leaves

Anatomy of a Leaf Blade – thin, flat part of leaf –

maximum light absorption Blade attached to stem by petiole Outer covering of dermal tissue

Top and bottom covered by epidermis, tough irregular cells with thick outer walls

Covered by waxy cuticle – waterproof, prevents water loss

Anatomy of a Leaf Vascular tissues bundles into veins that

run from stem through leaf Palisade mesophyll beneath upper

epidermis – closely packed cells that absorb sunlight

Spongy mesophyll contains air spaces connected to stomata – small opening in epidermis allowing for gas exchange

Transpiration Mesophyll cell walls moist for easy

diffusion Water can evaporate from these

surfaces by transpiration May be replaced by water from xylem Cools leaves on hot days, but can

threaten survival

Gas Exchange Exchange gases between air spaces in

spongy mesophyll and exterior by opening stomata

Homeostasis If stomata were always open, too much

water would be lost to transpiration Open just enough to allow

photosynthesis Guard cells control opening and

closing of stomata, regulating movement of gases

Homeostasis When water is abundant, increaser in water

pressure in guard cells opens stoma by curving

When water is scarce, water pressure in guard cells drops and stoma closes

Stomata usually open during day, closed at night

Can be closed in bright sunlight or hot/dry conditions

Respond to environment

Transpiration and Wilting Osmotic pressure keeps leaves/stems

rigid Water loss due to transpiration can lead

to a loss of pressure in the cells

Adaptations of Leaves Pitcher plant – attract/digest insects to

obtain nitrogen Living stone (rock plant) – 2 leaves for

hot/dry conditions are round to minimize exposure to air, have very few stomata

Spruce – waxy epidermis, stomata sunken below leaf surface

Cactus – photosynthesis occurs in stems, leaves are thorns

Section 23-5Transport in Plants

Water Transport Combination of transpiration and

capillary action moves water through xylem

Water evaporates through stomata, leaf dries out, water is pulled up through xylem

How Cell Walls Pull Water Upward Water molecules

attracted to each other by cohesion – H bonds form between molecules

Water molecules bond to other substances by adhesion

How Cell Walls Pull Water Upward Capillary action is

the tendency of water to rise in a thin tube because of cohesion and adhesion

Thinner tube, higher water will rise

Putting it All Together Xylem tissue hollow, connected tubes

(tracheids and vessel elements Tubes are lined with cellulose cell walls

(adhesion) Transpiration removes water from the

exposed walls, adhesion pulls water from interior of leaf

Pull is powerful - extends down through tips of roots to the water in the soil

Nutrient Transport Pressure-flow hypothesis1. Membranes of sieve tube cells use active

transport to move sugars from cytoplasm into sieve tube itself

2. Water follows by osmosis, creating pressure at the source of the sugars

3. If a plant region has a need for sugars, they are actively pumped out of the tube and into the tissue - water leaves the tube via osmosis, reducing the pressure

Nutrient Transport Flow of nutrient-rich fluid from the

sources of sugars (source cells) to the places where sugars are used or stored (sink cells)

Flexibility in changing seasons